1. Field of the Invention
The present invention relates to a solid state image sensor. More particularly, the present invention relates to a solid state image sensor to be used in a digital still camera, a digital video camera or the like.
2. Related Background Art
Distance measurement techniques for auto-focusing (AF) operations of digital still cameras and digital video cameras are known. With regard to such distance measurement techniques for AF operations, U.S. Pat. No. 6,829,008 proposes a solid state image sensor having some of its pixels provided with a distance measuring function and adapted to measure distances by way of phase detection.
Phase detection is a technique of measuring a distance by comparing the images of light passing through different regions on the pupil of a camera lens and measuring the distance by triangular surveying by means of stereo images.
This technique realizes high speed and high precision AF because the technique does not require any operation of moving a lens for measuring a distance unlike conventional contrast measurement.
It also realizes real time AF when picking up a moving image.
According to U.S. Pat. No. 6,829,008, an aperture is formed between the microlens and the photoelectric conversion section of a solid state image sensor at a position eccentric relative to the optical center of the microlens. With this arrangement, light passing through specific regions on the pupil of a camera lens can be selectively led to the photoelectric conversion section.
However, the above-described arrangement of an aperture at an eccentric position according to U.S. Pat. No. 6,829,008 may not necessarily be satisfactory for improving the accuracy of distance measurement because fluxes of light cannot be separated sufficiently due to scattering of light at the wiring section of the solid state image sensor.
Additionally, a problem as pointed out below can arise when the arrangement of U.S. Pat. No. 6,829,008 is applied to a small solid state image sensor having a small pixel size.
As the pixel size is reduced, the F value of the microlens for leading light to the photoelectric conversion section increases to make the size of diffraction image substantially equal to the pixel size.
Then, light expands in pixels and fluxes of light cannot be separated sufficiently at the eccentric aperture so that the obtained results may not necessarily be satisfactory for improving the accuracy of distance measurement.